Oilwell boreholes must be cased before the well is completed. The casing string runs from the surface of the earth down to the production formation in most instances, and often an additional, shorter casing string is run from the surface of the ground several hundred feet downhole below the lowest aquifer. The casing is bonded to the borehole with cement and great care is taken to make certain that there is no possibility of one formation contaminating another formation.
The casing is usually run with a relatively small drilling rig, such as an Ingersoll Rand T5 rig, for example. Sometimes special casing rigs are employed, and at other times, the casing is run with a large drilling rig, which, of course, is very expensive.
Small drilling rigs which have a traveling powered head are limited to the length of casing which can be accommodated. Casing joints are manufactured especially for the small rig and this joint is identified as a Range 1 casing joint having a 20 foot length. Larger, more expensive rigs accommodate Range 2 casing joints which are about 30 feet in length.
Each joint of casing has a coupling member, called a collar, secured to one end of each joint. This provides the joint with a pin end and a box end. It is customary to lift the box end, that is, the end of the joint having the collar attached thereto, up into the rig, and thereafter lower the joint so that the pin end engages the box end of the joint which is already in the hole. The joint is screwed onto the upper end of the string and the entire string lowered into the borehole, with this action continuing until the well has been properly cased.
It is customary to screw a sub into the box end of the casing joint and attach the sub to the traveling head so that the joint can be lifted into the derrick where the pin end is subsequently stabbed into the box end of the string. A backup tool is generally used to prevent the casing string in the hole from rotating, while a set of specially designed power tongs are used to engage and screw the joint onto the upper end of the casing string. This operation requires a considerable number of roughnecks and represents one of the most dangerous aspects of running casing into the hole. The repeated use of the sub causes undesirable wear of the threads and preferably is to be avoided, because the worn threads sometime fail and drop the string into the hole, which is considered catastrophic in the oil patch.
It would be desirable to be able to provide a new system of running casing into the ground wherein a small drilling rig can accommodate casing longer than 20 feet in length. It would furthermore be desirable to be able to avoid damaging the threaded ends of the casing by lifting the casing into the derrick and thereafter rotating the new joint of casing without attachments being applied to the threads of the box end. It would especially be desirable if these attributes could be accomplished with a reduced work force and reduced time, thereby effecting a considerable savings. Such a desirable expedient is possible by the practice of the present invention.